feat: style admin site in dev mode

[jennifer-richards]

When in dev mode, applies styling to the Django admin site that resembles main site's dev mode. It also changes the admin title from "Django administration" to "Datatracker administration".

This introduces an admin app with an apps.py. This lets us override the title / header for the admin site without customizing more templates. In theory we could put the contents of the ietf/admin/ directory into an ietf/admin.py instead, but this does not play well with TastyPie's api autodiscovery. 😡

The CSS for this is a bit clunky - importing ietf.css to the admin does nasty things to its formatting. Instead, I've copied the explicit values for a few colors to try to keep the admin consistent with the overall scheme of the site. This is fragile and we could do it better, but I don't think it's worth much more effort.